externalforcing

External forcing refers to any influence on a dynamical system that originates outside the system and drives it away from its natural state. In climate science, external forcing describes inputs that alter Earth’s energy balance, rather than results of the climate system’s internal dynamics. A common related concept is radiative forcing, the change in the net downward radiative flux at the top of the atmosphere caused by a forcing agent, typically expressed in watts per square meter (W/m^2).

Examples include natural forcings such as variations in solar irradiance and volcanic eruptions, and anthropogenic forcings such as increased greenhouse gas concentrations, sulfate aerosols, and land-use changes. Greenhouse gases raise the baseline energy in the system; aerosols can either cool or warm locally, and volcanic aerosols generally produce a short-term cooling by reflecting sunlight.

In mathematical models, external forcing is an input term added to the equations governing the system’s evolution. For example, a simple energy balance model may include a term F_ext(t) representing the time-varying external forcing; the system’s response depends on the forcing history and the model’s feedbacks (such as albedo, water vapor, clouds).

External forcing is distinct from internal variability, which arises from nonlinear interactions within the system. However, external forcing can influence the amplitude and pattern of internal variability and trigger transitions between states when thresholds exist.

Limitations: radiative forcing is a simplified, model-dependent metric; translating forcing into outcomes requires knowledge of feedbacks and regional factors. The term is used across disciplines, including climatology, oceanography, and nonlinear dynamics.